Wireless networks and wireless communications have become prevalent throughout society. This has created the need for increased capacity as well as faster and more reliable wireless communication techniques. A difficulty in wireless communications is interference caused by high speed signals from, for example and not limited to, notebook personal computers (PCs) causing interference with wireless devices such as, for example, and not limited to wireless local area networks (wireless LANs), wireless wide area networks (WWANs), and/or other wireless networks embedded, for example, in a mobile platform.
Certain signals generated in a notebook computer, for example, have been found to be serious sources of radio frequency interference (RFI). In particular, clocking signals used to transfer data to and from various portions of the PC (for example, between a processor and a memory) have been found to be sources of electromagnetic interference (EMI).
Spread spectrum clocking is a technique for reducing the average amplitude of clock harmonics by spreading the energy at a fundamental clock frequency over a small frequency interval. Although this technique is effective at reducing interference to radio receivers, it is not sufficient in many important cases, such as for example and not limited to, a global positioning system (GPS) on small form factor devices.
The present inventors have noticed that current tone suppression techniques such as tone excision, notch filtering, and adaptive filtering assume that the tonal interferer has a fixed frequency or is slowly time varying. They do not exploit the known structure of spread clock harmonics and, therefore, end up unnecessarily removing much of the signal energy that is near the interferer.
U.S. Pat. No. 7,279,989 issued on Oct. 9, 2007 to Bettner et al. and assigned to Intel Corporation addresses the problem of RFI due to clock harmonics. This patent discloses clock noise mitigation using a frequency adaptive process.
U.S. patent application publication 2008/0081586 published on Apr. 3, 2008 to Sreerama et al. and assigned to Intel Corporation discloses the management of system clocks to reduce RFI. A frequency range of at least one active channel of at least one wireless communication RF band is identified, an overlap between the frequency range of the at least one active channel and the frequency range of the at least one clock harmonic is identified, and a fundamental frequency of the at least one clock is shifted to shift the frequency range of the at least one harmonic out of the frequency range of the at least one active channel.
An IEEE publication entitled “Performance Analysis of Instantaneous Frequency-Based Interference Excision Techniques in Spread Spectrum Communications” to Chensu Wang and Moeness G. Amin (IEEE Transactions on Signal Processing, Vol. 46, No. 1, January 1998) discloses using open-loop adaptive excision filters for mitigation in direct sequence spread spectrum communications.
The present inventors have identified a need for improved RFI suppression.